ABUJA, Dec 17 (Reuters) - Nigeria has slashed its forecast for economic growth in 2015 due to the plunge in global oil prices, the finance minister's budget presented to parliament on Wednesday showed.

The ministry now sees Africa's biggest economy growing 5.5 percent in 2015, down from an earlier projection of 6.4 percent. Oil accounts for the bulk of government revenue in Nigeria, but global crude prices have almost halved over the last six months.

The naira currency closed at a record low on Wednesday, down 3.6 percent on the day at 187.10 to the dollar, after the central bank said it would hold its last foreign exchange auction of the year on Wednesday, triggering strong demand from some importers. The currency rarely reacts to budget news.

The 4.3 trillion naira ($23 billion) budget is based on a benchmark oil price of $65 a barrel, down from $77.50 this year, and a significant cut on previous budgets, Finance Minister Ngozi Okonjo-Iweala said.

"This budget here has been a tough one. Nobody said things were going to be easy," she said later in a presentation on the budget at the finance ministry.

Revenue was seen at 3.6 trillion naira. The projected deficit for 2015 was 755 billion naira or 0.79 percent of GDP, low by global standards.

The oil production forecast was set at 2.27 million barrels per day, down slightly from this year's assumption of 2.38 million. The figures were in line with earlier comments by the minister this month.

Oil accounts for about 15 percent of Nigeria's gross domestic product but it makes up 75-80 percent of government revenues. Government finances have been hammered by the slide in world oil prices since June.

"The budget seeks to protect the average Nigerian and ... the key is that we focus on diversification of the economy," Okonjo-Iweala said. "This country is a non-oil country and ... want Nigerians to begin to think of the country in that way."

Yet efforts to diversify over the years have had mixed results, and oil dependency is seen as the Nigerian economy's biggest systemic flaw.

TURBULENCE AHEAD

Global investors have taken a growing interest in Nigeria, but they worry about its tendency to squander its oil windfall on bloated government spending and patronage.

As usual the cost of running the government, difficult to cut at the best of times, makes up the lion's share of spending in 2015, some 2.6 trillion naira, against 634 billion of capital spending, which is easier to cut as it is often not fully implemented.

Okonjo-Iweala listed ways in which the treasury would attempt to claw back some extra non-oil revenue. These included stopping abuses of import waivers to save 36 billion naira, imposing a 10 percent surcharge on imports of new private jets, a 39 percent surcharge on luxury yachts and a 5 percent surcharge on luxury cars.

The government also aims to earn an additional 460 billion naira over the next three years by improving tax collection.

Since "all the price intelligence we have gathered from analysts predicts that the 2015 (oil price) will steady ... to around $65-$70 per barrel," there was no need to further cut the benchmark, she said.

The finance minister insisted Nigeria's debt profile remained healthy, with domestic borrowing around 570 billion naira, about the same as this year's. The country's combined domestic and external debt was 13 percent of GDP, or about $60.1 billion and $9.5 billion, respectively, she said.

"We have these difficult times but we are not trying to borrow our way out," she said. (Additional reporting by Camillus Eboh in Abuja and Oludare Mayowa in Lagos; Writing by Tim Cocks; Editing by Giles Elgood and Hugh Lawson)